Ophthalmologists and optometrists would like to have accurate representations of portions of subjects' eyes. Such representations include, for example, representations of a subject's corneal surfaces, corneal thickness, corneal density and lens surfaces. This information may be used, for example, to prescribe contact lenses, intraocular lenses and eye glasses, and to reshape the cornea by surgical procedures or to perform other surgical procedures. Since it is not comfortable to measure these data by physically contacting an eye, remote sensing techniques are preferably used to obtain the representations.
One common technique for obtaining representations of eyes includes projecting narrow bands of light (commonly referred to as slits or slit beams) onto a subject's cornea at multiple locations on the cornea. For each of the slits, after the light in the slit has been scattered by the eye, an image of a cross section of the eye is obtained using the scattered light. Images from tens of slit projections (e.g., approximately 40 slits of light at different locations) are used to construct representations of one or more portions of the subject's eye.
FIGS. 1 and 2 illustrate one type of measurement apparatus 100 in which slits of light S, S′, at various angular deviations (a) about an instrument axis 102, are projected. The slits impinge on multiple locations on a cornea C. FIG. 2 is a view of apparatus 100 taken along line 2-2 of FIG. 1. Light scattered by the eye from each slit permits a cross section of the eye to be obtained; and multiple cross sections from slits of different angular deviations permit two-dimensional or three-dimensional representations of the eye to be constructed.
To produce slits of light S, S′, a projector 122 having a long, thin aperture 110 (having a length extending in the Y direction in FIG. 1) placed in front of a source 120 projects slits of light onto a beam splitter (or mirror) 125 which directs the light onto cornea C and lens L along instrument axis 102. To achieve slits of light S and S′ at the various angular deviations, apparatus 100 (including all components therein) and a portion 170a of the front faceplate 170 of the apparatus are rotated about instrument axis 102. After the light is scattered by the eye, the scattered light re-enters the apparatus through a camera port 135 and is imaged by a camera 142 comprising a lens 130 and a CCD sensor 140. The light gathered by lens 130 is projected onto CCD sensor 140. One image is obtained for each of a plurality of rotational positions of the apparatus.
Although camera 142 and slit projection optics are arranged so that camera 142 receives diffuse scattering of light, a troublesome affect occurs when spurious portions of slit light are specularly reflected from the eye and are received by the camera. For example, such specular reflections may occur due to the structure of cells in an eye or due to the topography of a surface of the eye.